Hydrogen sulfide is a highly toxic, flammable gas that is present in many industrial process environments. Seriously toxic levels occur above 100 parts per million (ppm) in air and concentrations above 1000 ppm can be fatal. Hydrogen sulfide gas is a dense gas that can accumulate in low lying areas. Hydrogen sulfide is highly flammable and forms explosive mixtures with air over a wide range of concentrations. While hydrogen sulfide gas produces a strong odor of rotten eggs at low concentrations that readily warns of its presence, at higher concentrations or after long periods of exposure to low concentrations of this gas, the human olfactory senses are dulled and dangerously high concentrations may go undetected by the nose.